In the application of electric motors to operate various types of loads, it is not uncommon to match a load to equal very closely the starting torque of the motor. If the motor is of a permanent magnet, alternating current, synchronous bi-directional type, it has been found that the motor may start in a wrong direction under one or more of the following conditions:
(a) Increase or decrease of the value of the phase shift capacitor. PA1 (b) Increase of the alternating current power voltage. PA1 (c) Change in the alternating current power frequency. PA1 (d) Increase of the load in the direction of the wrong rotation. PA1 (a) Reduce the tolerance of the alternating current phase shift capacitor (e.g..+-.5%), which is expensive for an alternating current electrolytic capacitor. PA1 (b) Restrict the allowabble range of the power, voltage, and frequency, which severly limits the application of the motor. PA1 (c) Introduce a resistance in the coil or phase shift capacitor circuit, which reduces the efficiency of the motor.
Whether the motor will start in the right or wrong direction is usually random with the probability of the wrong direction increasing with the severity of the above conditions. Starting in the wrong direction usually occurs upon a power turn "on", a change in direction, or after incountering an excessive load or an abrupt stall.
Application of permanent magnet bi-directional electric motors to drive loads, such as ventilating dampers, is quite common. It is desirable to use as small a motor as possible to drive the damper. As such, a particular damper will normally be matched with a motor that has just a sufficient operating torque to correctly operate the damper. Sometimes a damper will become wedged or loaded so that the motor can not easily operate the damper in the desired direction. When this occurs, the damper motor reverses its operation, thereby operating in the wrong direction. This type of operation can be very detrimental and normally motors have been oversized in order to avoid this problem. Oversizing of motors is a rather expensive expedient.
Some methods used to insure directional reliability have been to:
Other solutions to this problem have been to provide mechanical no-back types of structures, or various types of lost motion arrangements. This involves retooling existing motors and can be rather expensive, even though the expedients used are very simple.